Gas turbines engines and power generation, such as the General Electric LM2500, are used in numerous marine applications and for power generation. This harsh environment demands early detection and analysis of impending turbine failure in order to prevent catastrophic failures which may endanger personnel as well as shipboard equipment.
The current "state-of-the-art" monitoring equipment incorporates the Wigner-Ville Distribution (WVD). The WVD monitors the once-per-rev vibration energy of the turbine components.
The once-per-rev energy occurs at the frequency corresponding to the rotational velocity of the turbine. This type of system is being used by the US and Foreign navies to monitor the vibration of the LM2500 gas turbine.
The existing system incorporates acceleration sensors that are mechanically connected to the turbine casing. These sensors provide an analog signals suitable for analysis. These signals are then fed into a data acquisition system to filter and digitize the signal.
Sensor output signals are converted to the frequency domain. In one aspect, the conversion can be implemented using Fast Fourier Transform-type (FFT) processing. The frequency domain representation can be processed by deleting those frequency components known to be of little or no interest.
In addition, the amplitudes of negative frequency components can be set to zero. Inverse FFT processing can then be carried out to produce a time domain signal having only frequencies of interest and no negative frequencies.
The technique of isolating sections of the frequency domain signal to create only a few frequency components in the signal allows the WVD to be applied without any smoothing to reduce cross-term energy. The cross-term energy was controlled to occur in spectral locations that were not used in the analysis. This technique can be used in other analysis efforts with the WVD.
The WVD thus provides a highly accurate measure of the turbine vibration amplitude at any instant. This has been used as an important feature in a condition-based monitoring application.
As the vibration level changes under identical operating conditions, the change can be recorded and used as a measure of turbine health. Steadily increasing vibration levels indicate deteriorating turbine health.
The output of the WVD can be used with a thresholding algorithm to detect excessive once-per-rev (1X) vibration. Thresholding of the 1X vibration is used in many gas turbine installations. This allowed detection of deteriorating turbine condition. Ideally, the deterioration would be detected at an early enough stage to prevent catastrophic damage, and to schedule maintenance activities.
Complex signals have been processed by neural networks. One known form of a neural net is the Fuzzy Adaptive Resonance Theory, Fuzzy (ART) neural network.
This is a neural network architecture developed by Stephen Grossberg and Gail A. Carpenter of the Department of Cognitive and Neural Systems at Boston University. The network uses a resonance concept that involves comparisons of new inputs to information that is already learned.
If the new input is sufficiently close to the old information, then resonance occurs and the network will adapt to learn the new information. The first implementations of ART operated only on binary (0 or 1) data. The theory has been extended to accommodate analog input values of the range zero to one, using Fuzzy ART. The "Fuzzy" prefix implies that the input numbers are analog; no fuzzy logic connotations need be ascribed to the numbers. The system learns to recognize analog values within subsets of its total input space. The information that is presented to the network occupies a section of the input space corresponding to the amplitude of the information.
Fuzzy ART is called an unsupervised neural network because the training set is presented to the network without information as to the desired classification. Instead, the network forms an internal representation of the data presented to it.
While the known vibration analysis systems have generally been effective for their intended purpose, it would be desirable to improve the accuracy and extent of the analysis. In this regard, it would be desirable to be able to detect the presence of non-stationary frequency components. It would also be desirable to be able to eliminate frequency components not of interest.
It would also be desirable to be able to characterize, in some sense, a particular turbine and then monitor, over time the performance of that unit. In this fashion, deviations from expected performance should be detectable at an early enough stage to avoid the occurrence of catastrophic failures. Detected deteriorating performance can trigger unit maintenance.